This invention relates to the production of fused silica glass, and, in particular, to methods and apparatus for improving the homogeneity of such glass, i.e., for reducing variations in the index of refraction of the glass.
FIG. 1 shows a prior art furnace 100 for producing fused silica glass. In overview, high purity fused silica glass is made by depositing fine particles of silica in a refactory furnace at temperature exceeding 1650xc2x0 C. The silica particles are generated in a flame when a silicon containing raw material along with natural gas is passed through a burner into the furnace chamber. These particles are deposited on the hot surface of a rotating body where they consolidate into a very viscous fluid which is later cooled to the glassy (solid) state. The rotating body is in the form of a refractory cup or containment vessel which is used to provide insulation to the glass as it builds up, so that the furnace cavity formed by the cup interior and the crown of the furnace is kept at high temperatures. In the art, glass making procedures of this type are known as vapor phase hydrolysis-oxidation processes or simply as flame hydrolysis processes. The body formed by the deposited particles is often referred to as a xe2x80x9cboulexe2x80x9d and this terminology is used herein, it being understood that the term includes any silica containing body formed by a flame hydrolysis process.
The furnace 100 includes a crown 12 having multiple deposition burners 14, a ring wall 160 which supports the crown, and a rotatable base 18 mounted on an oscillation table 20. The base 18 is rotatable about an axis 3, and the table 20 oscillates in a x-y direction in a plane perpendicular to the axis 3. The crown, ring wall, and base are each made of suitable refractory materials. Preferred patterns for the motion of the x-y oscillation table 20, which can be used in the practice of the present invention, are described in commonly assigned U.S. Pat. No. 5,696,038, entitled xe2x80x9cBOULE OSCILLATION PATTERNS OF PRODUCING FUSED SILICA GLASSxe2x80x9d.
A cup or containment vessel 13 is formed on the base 18 by means of a cup wall or containment wall 22 mounted on the base 18, which forms the cup or containment vessel 13. The cup or containment wall 22 and the portion of the base 18 surrounded by the wall (the bottom of the vessel) is covered with high purity bait sand 24 which collects the initial silica particles. The wall 22 can be composed of refractory blocks such as outer alumina base block 22a and an inner liner 22b made of, for example, zironcia or zircon. Other refactory materials and constructions can, of course, be used if desired.
Surrounding the cup wall 22 of the cup or containment vessel 13 is a shadow wall or air flow wall 130. The shadow wall 130 is mounted on x-y oscillation table 20 by means of feet 140, e.g., by four feet equally spaced around the circumference of the shadow or air flow wall. Other means of mounting the air flow wall to the oscillation table can be used if desired. In general, the mounting means should include spaces for the ingress of air to the space 175 between the cup or containment wall 22 and the shadow or air flow wall 130.
Surrounding the shadow wall 130 is a stationary ring wall 160 which supports the crown 12. A seal 155 is provided between the stationary ring wall 160 and the rotatable and oscillatable shadow or air flow wall 130. The seal 155 comprises an annular plate 150 which rides in or slides in an annular channel 170 formed within the stationary ring wall 160. The annular channel 170 can comprise a C-shaped annular metal plate which forms the bottom of the stationary wall, or other forms of motion accommodating seals can be used if desired, including flexible seals composed of flexible metal or refractory cloth which, for example, can be in the form of a bellows.
The furnace of FIG. 1 employs two gaps around the cup-like containment vessel 13, including a circumferential gap or passage 175 between the containment wall 22 and the shadow or air flow wall 130, which gap permits the flow of cooling air along arrows a into the plenum 26 formed between the crown 12 and the vessel 13. The other gap 165 is formed between the air flow wall 130 and the stationary ring wall 160, and has a variable dimension resulting from the oscillation of the table 20, but does not permit the flow of air as a result of the motion accommodating seal 155. Thus, the air flow around the boule 19 is influenced by the infiltrated air through passage 175 and the inflow from burners 14.
The products of combustion from burners 14 are exhausted through six ports such as 280, that are built around the furnace. As noted in FIG. 1, the furnace is built such that there are three layers of refractory wall between the glass boule 19 and the ambient air. The innermost wall 22, which is part of the cup-like vessel 13, is isolated from the boule by liners 22b such as zircon. The second layer of wall, termed the shadow or air flow wall 130, is separated from the cup-like vessel 13 by a gap of roughly three inches. The outermost layer of wall, termed the stationary ring wall 160, is further separated from the shadow wall 130 by an air gap 165 that roughly measures four inches. The walls are built to provide a furnace cavity where the temperatures as well as the furnace atmosphere can be maintained. Radial and circumferential uniformity of both furnace atmosphere and the temperature is important because it directly effects the quality of glass. Temperature uniformity is important for providing consistency in glass density and refractive index. Compositional uniformity is important in providing the consistency in glass density and the hydrogen dissolved in the boule 19.
Although the horizontal steel plate 150 effectively blocks the gap 165 between the ring wall 160 and the shadow wall 130, the gap 175 between the shadow or air flow wall 130 and the cup containment wall 22 is open so that the ambient air is free to flow from close to the furnace base from the furnace room to the exhaust ports 280. This air flow through passage 175 and into the chamber or plenum 26 of the furnace is necessary to cool down the steel bands which hold the refractory blocks together forming the cup like vessel 13. Without the benefit of such cooling air, the steel bands would expand and slip out of retaining grooves resulting in the vessel 13 falling into pieces.
In order to provide effective removal of the products of combustion from the furnace cavity or plenum 26, the six port boxes 280 are maintained at a negative pressure. Because air is free to move through the circumferential passage 175 between the cup or containment wall 22 and the shadow or air flow wall 130, the furnace exhaust consists of gases from burners 14 pulled out of the furnace cavity 26 and the air pulled through the gap or passageway 175 as indicated by arrows a. Because of the flow patterns created near the rim of the cup vessel 13 it became clear that a portion of the air that was being pulled up through the passage 175 was in fact entering the furnace cavity 26. Gas composition measurements carried out at various radial depths from the inner surface of wall 22, indicated that the products of combustion were being diluted by the infiltrated air, but that the concentration of CO2 increased as the sampling probe was inserted radially deeper into the furnace cavity. Thus, it became apparent that the effect of the infiltrated air flow through passage 175 on the homogeneity of the boule 19 was that rim portions were influenced to a greater degree than central portions. The adverse effect of the entrained air through passage 175 is twofold. Firstly, it cools the products of combustion near the rim of the vessel 13 and reduces the gas temperatures in such region. Secondly, the effect is the dilution of the furnace atmosphere which results in the glass close to the outer periphery of the vessel 13 having a different refractive index and less amount of hydrogen dissolved than centrally of the boule. If the glass is cooled by a large magnitude, the result is in an opaque glass on the outside.
In view of the foregoing, it is an object of this invention to provide improved methods and apparatus for producing silica-containing boules by the flame hydrolysis. In particular it is an object of the invention to improve the homogeneity of such boules. It is a further object of the invention to provide an improved cup or containment vessel design which will enhance uniformity in the refractive index and the dissolved hydrogen in the boule formed therein.